Heretofore, single and double acting hydraulically operated cylinders have been widely used for lifting and lowering loads relative to load supports of various types.
While, for many installations, conventional cylinders perform satisfactorily, there are two instances where serious installation and/or operating problems are encountered. A first instance involves an installation, wherein a single cylinder is employed to lift a relatively heavy load, such as the door or shell of a pressure vessel, wherein the path of movement of such load is offset from the path of movement or axis of the piston rod of the cylinder. Problems of bending deformation of such piston rod increase with increases in the weight of the load, the distance between the paths of movement of the load and piston rod and the required length of piston rod travel. Solutions to such problems commonly include the provision of over-size cylinders, provision of special guides to constrain movement of the load to its prescribed path of travel and attempts to align the paths of travel of the load and piston rod.
A second instance involves an installation, wherein the weight of a load is such as to require that two or more cylinders be arranged in parallel and be synchronously operated. The need for providing for like movements or displacements of the piston rods of paired cylinders in order to prevent canting of the load and/or unequal bending of the piston rods is well recognized, but impossible to accomplish in many installations even when the hydraulic fluid control circuit is provided with special flow control valves in an effort to achieve balanced operation conditions in the cylinders.